1. Field of the Invention
This invention relates in general to short-range radars, and in particular, to transceivers of automotive radars of the Precise Positioning System (PPS) which is intended to be a constituent of the Intelligent Vehicle Initiative (IVI) of the U.S. government. Once realized, this system will provide a determination of a safe and precise location of a vehicle in a traffic lane on the road, prevent traffic accidents when the vehicle is exiting the roadway and otherwise reduce the likelihood of traffic accidents. The automotive radars serve as sensors of the PPS that provide its secure and efficient round-the-clock functioning under any weather conditions.
2. Description of the Prior Art
A crucial section of an automotive millimeter-wave (MM-wave) radar sensor is a transceiver front-end whose key element is a source of electromagnetic radiation (EMR). The most significant requirements for it are the levels of phase noise and power output. Their required values are obtained, with minimal over-all dimensions and costs, in the transceiver circuit of continuous wave radar with linear frequency modulation (FMCW radar).
When choosing the final construction of the on-board radar front-end, the purpose-oriented criterion (from the viewpoint of mass production profitability) should be maximal simplicity of the circuit at a level of operating characteristics required for a particular application. At present, it is the Gunn diode (GD) oscillators (whose frequency is stabilized with a high-Q cavity resonator) that meet most completely the set of requirements for EMR sources of on-board radars operating in the 76-77 GHz frequency range. In actual front-ends, the frequency-setting elements are voltage controlled oscillators (VCO) with indium phosphide [see, D. D. Li, S. C. Luo, C. Pero, X. Wu and R. M. Knox, Millimeter-wave FMCW/monopulse radar front-end for automotive applications, IEEE MTT-S Intern. Microwave Symp. Digest. 1999, vol. 1, pp. 277-280; M. E. Russell, A. Crain, A. Curren et al. Millimeter-wave radar sensor for automotive intelligent cruise control (ICC), IEEE Transactions on Microwave Theory and Techniques. 1997, vol. 45, no. 12, pp. 2444-2453] or gallium arsenide [see, L. P. Lowbridge, Low cost millimeter-wave radar systems for intelligent vehicle cruise control applications, Microwave Journal, 1995, vol. 38, no. 10, pp. 20-33] GDs operating at the fundamental (second) harmonic. The reasons for choosing GD oscillators are as follows:                there exists corresponding well-refined efficient manufacturing technology;        the power output of such diodes is quite sufficient for automotive radars;        such oscillators can be easily retuned in the required frequency range, and        they have good noise characteristics.The typical specifications of such oscillator in the frequency range of 76-77 GHz (GaAs GD) are as follows: power output of 30 mW in the electric frequency tuning band of about 1000 MHz, with phase noise value better than −85 dBc/Hz (relative to the carrier frequency level) at tuning away the carrier frequency by 100 kHz [see, L. P. Lowbridge, Low cost millimeter-wave radar systems for intelligent vehicle cruise control applications, Microwave Journal. 1995, vol. 38, no. 10, pp. 20-33]. It should be noted that, although the GD oscillators can provide the required power output and have quite satisfactory noise characteristics, the cost of assembling and tuning wave-guide diode oscillators is very high. In addition, their efficiency is low (about 2%-4%), while the volume physical structure of diodes hampers realization of transceiver construction as a planar structure.        
In most modem automotive MM-wave radars, the transceivers (front-ends) are made using hybrid-integrated technology. Specifically, a parabolic or lens antenna is made with a waveguide feed, and diode (most often GD) oscillators in a cavity resonator are used as EMR source. The rest of front-end components and circuits are usually made as hybrid sections, using the elements of manufacturing technology for microstrip, coplanar or fin-lines.
At present, due to considerable advances in development of semiconductor manufacturing technology, as well as assembling and testing techniques, the following EMR sources are intensely being developed and planned for realization in the constructions of the next-generation on-board radars: MM-wave oscillators made as monolithic integrated circuits (MIC) (multi- or one-chip) on the basis of pseudomorphic high-electron-mobility transistors (PHEMTs) or heterojunction bipolar transistors (HBTs) [see, L. Raffaelli, Millimeter-wave automotive radars and related technology, IEEE MTT-S Intern. Microwave Symp. Digest. 1996, TU1B-2, pp. 35-38; I. Gresham, N. Jain, T. Budka et al., A compact manufacturable 76-77-GHz radar module for commercial ACC applications, IEEE Transactions on Microwave Theory and Techniques, 2001, vol. 49, no. 1, pp. 44-58]. However, the specifications of the present-day MICs operating in the 76-77 GHz frequency range still do not meet the imposed requirements. The problems of integration and packaging of active devices, as well as their mechanical and electric coupling with each other and the rest of transceiver components, still remain largely unsolved and potentially expensive. However, upgrading manufacturing technology and improving specifications of MM-wave MICs are being worked on. This should make it possible to apply them in the on-board radar systems of the next generation [see, I. Gresham, N. Jain, T. Budka et al., A compact manufacturable 76-77-GHz radar module for commercial ACC applications, IEEE Transactions on Microwave Theory and Techniques, 2001, vol. 49, no. 1, pp. 44-58; M. Vossiek, T. v. Kerssenbrock and P. Heide, Novel nonlinear FMCW radar for precise distance and velocity measurements. IEEE MTT-S Intern. Microwave Symp. Digest. 1998, vol. II, pp. 511-514].
In recent years, a more promising approach to the development of radar transceiver seems to be solving the problem of generating MM-wave electromagnetic oscillations at lower frequencies, with further frequency conversion into the 76-77 GHz range with frequency multipliers. Conversion of a microwave frequency-modulated signal with the required conversion characteristics (e.g., conversion range and linearity) into the MM-wave range by frequency multiplication is made without appreciable phase distortions. This makes it possible to obtain a MM-wave signal with a phase noise level essentially below that of diode and transistor active oscillators operating at the fundamental frequency. Specifically, one obtains the required level of phase noise, high temperature stability of oscillation frequency and good isolation from load (low pulling figure). In addition, the manufacturing technology for microwave components (that has been well refined in the microwave range) provides device availability and rather low cost. The inexpensive starting materials (involving also semiconductor discrete devices), as well as available and well-matured techniques for device assembling and testing at mass production, add attraction to this procedure of development and production of stable oscillators in promising MM-wave regions [see, I. Gresham, N. Jain, T. Budka et al., A compact manufacturable 76-77-GHz radar module for commercial ACC applications, IEEE Transactions on Microwave Theory and Techniques, 2001, vol. 49, no. 1, pp. 44-58].
The combination of a sufficiently high power output and acceptable conversion losses with low level of intrinsic phase noises makes frequency multipliers a very promising component in transceivers of MM-wave systems, in particular, in automotive radars. Frequency multipliers are successfully applied here in circuits where probing signals of transmitting facilities are formed, as well as in those circuits where signals of receiver local oscillators are formed. It is believed that application of frequency multipliers is especially efficient in those systems where the crucial requirement is a combination of high specifications (in particular, phase noise level), high reliability and reasonable cost [see, H. Bierman, Innovative circuit arrangements and device designs provide high performance RF and MM-wave sources for military applications, Microwave Journal, 1989, vol. 32, no. 6, pp. 26-42; D. F. Peterson, The varactor power frequency multiplier, Microwave Journal, 1990, vol. 33, no. 5, pp. 135-146.].
Among the known ways of developing diode frequency multipliers (those on the basis of nonlinear dependencies of diode reactive parameters on voltage; GD and TUNNET-diode harmonic oscillators), one should particularly note high-factor multipliers with impact avalanche transit time (IMPATT) diodes. The traditional frequency multiplication techniques are efficient at factors of about 2-3 only, at which a relatively large number of multiplication stages (and, accordingly, intermediate amplifiers) are required to provide highly efficient MM-wave EMR sources when multiplying signals from quartz-stabilized oscillators. By contrast, application of IMPATT frequency multipliers enables one to obtain a highly stable low-noise MM-wave signal using a minimal number of active elements.
The efficiency of IMPATT diode frequency multiplier has been demonstrated, both theoretically and experimentally, in the MM-wave range, because it has small conversion losses at high frequency multiplication factor. In particular, it was found that in this mode the output signal power level at the n-th harmonic is proportional to 1/n [see, P. A. Rolland, E. Constant, A. Derycke, J. Michel, Multiplication de frequence par diode avalanche en ondes millimetriques, Acta Electronica, 1974, vol. 17, no 4, pp. 213-228; P. A. Rolland, J. L. Waterkowski, E. Constant, G. Salmer, New modes of operation for avalanche diodes: frequency multiplication and upconversion, IEEE Trans. on Microwave Theory and Techniques, 1976, vol. MTT-24, no. 11, pp. 768-775]. This is significantly more than the attainable level for charge-storage diode multipliers where the corresponding dependence is 1/n2 [see, A. I. Sobolev, Yu. A. Kotov, L. A. Modestov, Superhigh-ratio frequency multipliers. In: “Semiconductor Devices and Their Application”, No. 23, Sovetskoe Radio Publ. Moscow. 1970, pp. 109-132 (in Russian)].
The circuit of MM-wave automotive radar transceiver with high-factor frequency multipliers has a number of advantages over those where EMR source is an oscillator operating at the basic frequency (e.g., a GD oscillator with varactor frequency tuning [see, L. H. Eriksson, Automotive radar for adaptive cruise control and collision warning/avoidance, Radar 97, Proceedings of Radar Systems, (Conf. Publ. No. 449). 1997, pp. 16-20] or a FET oscillator [see, U.S. Pat No. 4,931,799 entitled Short-range radar transceiver employing a FET oscillator”]). In particular, the problem of highly linear frequency tuning is transferred into the microwave range where standard, available and reliable transistor VCOs are used. High linearity of these VCOs enables one to apply linearization circuits without feedback. This provides considerable reduction of the whole system cost, as well as improvement of its reliability and operating speed.
It is also known that the phase noise level is a very important parameter in many applications. Realization of EMR sources with reasonable values of this parameter (especially in the short-wave region of the MM-wave range) creates a difficult problem. Traditionally, this is achieved by introducing phase synchronization of an oscillator by a highly stable low-frequency source [see, A. D. Patsyuk, Sources of high-frequency oscillations for MM-wave systems, Zarubezhnaya Radioelektronika, 1988, no. 11, pp. 79-86 (in Russian)]. However, the number of active devices (and, as a result, the cost of the whole system) grows considerably. In addition, one should note a rather low temperature stability of the frequency of oscillators operating in that region of the MM-wave range, so that rather complex thermocompensation systems have to be applied for their reliable functioning [see, H. H. Meinel, Automotive radar and related traffic applications of millimeterwaves, 1997 Topical Symposium on Millimeter Waves, 1998, pp. 151-154].
Thus, use of such an IMPATT frequency multiplier with a high multiplication factor in a transceiver of an on-board radar enables (using a single component) transfer of a highly stable signal (formed in the microwave range) into the MM-wave range without appreciable phase distortions. Moreover, the efficiency of application of IMPATT frequency multipliers increases at their use in both the circuits of formation of transmitter probing signals and those of receiver local oscillator (LO) signals.